The present invention relates to improvements in injection pumps for diesel engines, with electronic output regulation.
Pumps of this type are already known in which an adjusting screw of at least one pumping element is slidable in a seating machined in the pump body and connected to a feed chamber adapted to receive fuel under pressure from an auxiliary pump, the rotation of the screw being regulated by a control rod which is displaceable longitudinally by proportional electrohydraulic actuator means carried on the pump body, the said actuator means including a distributor valve governed by an electromagnet which controls the feed and exhaust of the fluid under pressure respectively to and from an actuator cylinder containing a piston connected to the control rod, so as to obtain for each value of an electrical command signal from an electronic pump control unit a given position of the said piston and therefore of the control rod.
The desired linear positioning of the piston and therefore of the control rod and the angular positioning of the pumping element are ensured by a detector mounted to one of the said positioning elements. This detector is adapted to produce a feedback signal which is constantly compared with the said command signal so that the electromagnet receives, when the said two signals are equivalent to each other, a signal in response to which the distributor valve assumes a position in which communication between the actuator cylinder and the fluid feed and exhaust respectively is interrupted.
In known injection pumps of the type specified above, the actuator cylinder and the distributor valve are disposed in a housing separate from the pump body in which the pumping element or elements is or are mounted, this housing being fixed to the pump body. The electromagnet is also carried on the said separate housing in which the actuator cylinder and the distributor are located. Such a construction necessitates a mechanical linkage between the control rod and the piston of the hydraulic cylinder, and it is impossible in practice to ensure a perfect concentricity between the seatings in which the rod is guided and the actuator cylinder in which the respective piston slides to control the displacements of the said rod. Such a mechanical linkage between the piston and the control rod is a source of play and friction, as well as of forces perpendicular to the axis of the rod, which forces increase the friction and give rise to a tendency for the rod to stick, particularly when alignment defects exist between the respective seatings and the actuator cylinder. Consequently an effective delivery control may be obtained only with fairly high pressures which may only be obtained with the use of expensive separate hydraulic control units for feeding fluid under pressure to the distributor valve.
Moreover, the pump may be provided with a safety stop device which includes a thrust rod facing the end of the control rod opposite the piston of the actuator cylinder, and is hand operated with servo assistance when the fuel feed pressure at the injection pump inlet drops, to avoid the residual fuel present in the feed chamber from being sent to the injectors with the danger of continuing to rotate the engine in the stopping stage. In this case the separate hydraulic control must also include solenoid valves and the electronic pump control circuit will necessarily be more complicated.
The present invention has the object of avoiding the above-mentioned disadvantages.